Surgical mesh initially used for hernia and abdominal wall defects are now being used for other types of tissue repair, such as rotator cuff repair, pelvic floor dysfunction, and reconstructive or cosmetic surgeries. It is projected that in 2010, there will be more than 8 million hernia procedures, 800,000 rotator cuff repairs, 3 million pelvic prolapse repairs, 600,000 urinary incontinence repairs, and 1.5 million reconstructive or aesthetic plastic surgeries. Most of these procedures will likely employ implantable surgical mesh devices currently on the market, including: Bard Mesh (polypropylene) by C. R. Bard; Dexon (polyglycolic acid) by Synecture/US Surgical; Gore-Tex (polytetrafluoroethylene) by W. L. Gore; Prolene (polypropylene), Prolene Soft (polypropylene), Mersilene Mesh (polyester), Gynemesh (polypropylene), Vicryl Knitted Mesh (polyglactin 910), TVT (polypropylene) by Ethicon; Sparc tape (polypropylene) by American Medical Systems; and IVS tape (polypropylene) by TYCO Healthcare International.
Surgical mesh devices are typically biocompatible and may be formed from bioresorbable materials and/or non-bioresorbable materials. For example, polypropylene, polyester, and polytetrafluoroethylene (PTFE) are biocompatible and non-bioresorbable, while polyglactin 910 and polyglycolic acid are biocompatible and bioresorbable.
Though current surgical mesh devices may be formed from different materials, they have various similar physical and mechanical characteristics beneficial for tissue repair. However, despite the benefits provided by current surgical mesh devices, their use may be accompanied by a variety of complications. Such complications, for example, may include scar encapsulation and tissue erosion, persistent infection, pain, and difficulties associated with revision surgery. In addition, the use of an absorbable material may result in reoccurrence due to rapid resorption of the implant material and loss of strength.
Although polypropylene monofilament may be a highly regarded material for surgical mesh devices, polypropylene mesh devices can induce intense scar formations and create a chronic foreign body reaction with the formation of a fibrous capsule, even years after implantation. Minor complaints of seromas, discomfort, and decreased wall mobility are frequent and observed in about half of the patients implanted with polypropylene mesh devices. Moreover, polypropylene generally cannot be placed next to the bowel due to the propensity of adhesion formation.
Although the use of multifilament polyester may improve conformity with the abdominal wall, it is also associated with a variety of disadvantages. For example, higher incidences of infection, enterocutaneous fistula formation, and small bowel obstruction have been reported with the use of multifilament polyester compared to other materials. Indeed, the small interstices of the multifilament yarn make it more susceptible to the occurrence of infection, and thus multifilament polyester is not commonly used within the United States.
The use of polytetrafluoroethylene (PTFE) may be advantageous in minimizing adhesions to the bowel. However, the host tissue encapsulates the PTFE mesh, resulting in weak in-growth in the abdominal wall and weaker hernia repair. This material, though not a good mesh material on its own, has found its place as an adhesion barrier.
Absorbable materials, such as Vicryl and Dexon, used for hernia repair have the advantage of being placed in direct contact with the bowel without adhesion or fistula formation. A study has observed that Vicryl has comparable burst strength to nonabsorbable mesh at three weeks but is significantly weaker at twelve weeks due to a quick absorption rate. Meanwhile, the same study observed that Dexon has more in-growth at twelve weeks with less absorption of the mesh. The concern with absorbable meshes is that the rate of absorption is variable, possibly leading to hernia recurrence if the proper amount of new tissue is not there to withstand the physiologic stresses placed on the hernia defect.
A significant characteristic of a biomaterial is its porosity, because porosity is the main determinant for tissue reaction. Pore sizes of >500-600 μm permit in-growth of soft tissue; pore sizes of >200-300 μm favor neo-vascularisation and allow mono-morphological restitution of bony defects; pore sizes of <200 μm are considered to be almost watertight, hindering liquid circulation at physiological pressures; and pores of <100 μm only lead to in-growth of single cell types instead of building new tissues. Finally, a pore size of <10 μm hinders any in-growth and increases the chance of infection, sinus tract formation, and encapsulation of the mesh. Bacteria averaging 1 μm in size can hide in the small interstices of the mesh and proliferate while protected from neutrophilic granulocytes averaging 10-15 μm.
Other important physical characteristics for surgical mesh devices include thickness, burst strength, and material stiffness. The thickness of surgical mesh devices vary according to the particular repair procedure. For example, current surgical mesh device hernia, pelvic floor dysfunction, and reconstructive/cosmetic procedures range in thickness from approximately 0.635 mm to 1.1 mm. For rotator cuff repair, a thickness of 0.4 mm to 5 mm is typically employed.
Intra-abdominal pressures of 10-16 N, with a mean distension of 11-32% results in the need for a surgical mesh with a burst strength that can resist the stress of the inner abdomen before healthy tissue comes into being.
Material stiffness is an important mechanical characteristic for surgical mesh, especially when used for pelvic floor dysfunction, because material stiffness has been associated with the likelihood of tissue erosion. Surgical mesh devices formed from TVT, IVS, Mersilene, Prolene, Gynemesh, Sparc tape, for example, currently have an ultimate tensile strength (UTS) that exceeds the forces exerted by intra-abdominal pressures of 10-16 N. With the low force in the abdomen, the initial stiffness of the material is an important consideration. Moreover, the stiffness may exhibit non-linear behavior most likely due to changes in the fabric structure, e.g., unraveling of the knit, weave, etc. A surgical mesh device of lesser stiffness may help reduce tissue erosion and may conform to the contours of the body more effectively.
With regard to soft tissue repair, in particular, surgical meshes and scaffolds have been used historically in a variety of applications to reinforce soft tissue where defects or weakness exist. They are widely used for breast and chest wall reconstruction, strengthening tissues, providing support for internal organs, and treating surgical or traumatic wounds. They are usually made of inert materials and polymers such as Teflon®, polypropylene, polyglycolic acid, polyester, polyglactin 910, etc., although a titanium mesh has been used in some spinal surgeries. But the use of tissue based materials such as acellular dermal matrix (ADM) from human and animal derived dermis is also becoming more popular.
Some examples of the surgical mesh devices that are currently available in the market are Bard Mesh by C. R. Bard; Dexon by Synecture/US surgical; Gore-Tex by W. L. Gore; Prolene, Prolene Soft, Mersilene Mesh, Gynemesh, Vicryl Knitted Mesh, and TVT by Ethicon. Some examples of the surgical acellular dermal matrix devices that are currently available are Alloderm and Strattice by Lifecell, FlexHD by Ethicon, DermaMatrix by Synthes and Permacol by Covidien.
Surgical mesh devices are typically biocompatible and can be made from bioresorbable and/or non-bioresorbable material. For example, Teflon®, polypropylene and polyester are biocompatible and non-bioresorbable while polyglycolic acid and polyglactin 910 are biocompatible and bioresorbable. ADM is processed by removing the cells and epidermis, if applicable, from the donor tissue and leaving only natural biologic components. The most common tissue based materials are human, porcine or equine derived dermal matrix.
One application for soft tissue reconstruction that uses surgical mesh or ADM is breast reconstruction post mastectomy. The aim of breast reconstructive surgery is to restore a woman's breasts to a near normal appearance and shape following the surgical removal of a breast (mastectomy), a crucial step towards emotional healing in women who have been faced with losing their breast as a result of a medical condition such as breast cancer. According to the American Society of Plastic Surgeons (ASPS), approximately 60,000 surgical procedures occur in the US related to non-cosmetic breast reconstruction. Internationally, that number surpasses 80,000 procedures when major industrialized countries are taken into consideration. A contoured, structural and tailored scaffold device designed to meet the unique needs of the breast reconstruction population where a massive loss of tissue occurs and which would work with the body's own immune process to restore the environment to a more natural state would provide a compassionate solution to a significant unmet need.
The breast reconstruction surgical procedure is commonly performed with two different methods, both using ADM as the preferred matrix. The main advantage of ADM over other available surgical meshes is the higher rate of revascularization, providing support and coverage of the defect while preventing infection and capsular contraction. The first method, a one stage reconstruction uses ADM to fully reconstruct the shape of the breast in conjunction with a breast implant at the time of the surgical procedure. The second method, a two stages reconstruction; the first stage consisting of the placement of (a) tissue expander(s) (at the time of mastectomy or later) with ADM to reconstruct the breast; follow by tissue expander expansion with saline solution to expand the muscle and skin tissue; the second and final stage consisting of the replacement of the tissue expander with an implant. In both procedures the pocket for the tissue expander or the implant is created by releasing the inferior origin of the pectoralis muscle and electrocauterizing a subpectoral pocket. A sheet of ADM is centered over the defect and it is sutured to the inframammary fold with continuous or interrupted sutures. The tissue expander or implant is inserted and positioned inside the subpectoralis pocket created. The rest of the ADM is cut to the necessary shape and it is sutured to the inferior edge of the pectoralis muscles, while on the lateral border is sutured to the pectoralis and serratus muscles.
A breast reconstruction procedure alternative to the one described above using ADM is performed with autologous tissue such as TRAM flap. In this surgical procedure, the breast is reconstructed by using a portion of the abdomen tissue group that has been surgically removed, including the skin, the adipose tissue, minor muscles and connective tissue. This abdomen tissue group is taken from the patient's abdomen and transplanted onto the breast site using a similar method as described above with ADM.
The quality of the resulting reconstruction is impacted by subsequent treatment, e.g. post-mastectomy radiation weakens skin tissue, the amount of tissue available e.g. thinner women often lack sufficient tissue, and the overall health and habits (smoking) of the individual. Tissue expanders, balloon type devices, are frequently used in an attempt to stretch the harvested skin to accommodate the breast implant. However, harvested tissue has limitations in its ability to conform to the natural breast contour resulting in unacceptable results, including a less than ideal positioning or feel of the breast implant. A scaffold device that can be used as an internal scaffold to act as a “bra” to immediately support a geometrically complex implantation site at the time of surgery would ideally provide the body both the time and structure necessary for optimal healing.
Breast reconstruction in two stages with a tissue expander and ADM followed by the replacement of the tissue expander with an implant has become the most common technique adopted by surgeons. A main advantage is the lengthening of the pectoralis major muscle therefore preventing the commonly referred to as “window shading” after the muscle is released. Another main advantage is the control of the inframammary fold position and shape as well as the lateral breast border.
The use of ADM has advantages against the common surgical mesh devices by lowering the rate of capsular contraction and infection; however despite its low overall complication rate, the procedure is not without risk since ADM can generate a host inflammatory reaction and sometimes present infection. Also, it is very important to note that the properties of ADM are limited to the properties of the tissue that is harvested which can result in variability.